CODEWITHSUNDEEP
javascriptnode.js
ambach 66
ambach 66

Reputation: 113

Javascript Promises syntax differences?

I am having little doubt with how javascript promises are used. I have learned it in a following basic format and it's understandable. 

var dopromise = new Promise (function(resolve,reject){

let isclean=true;

if(isclean){
   resolve('done');
 }

else
{
    reject();
}
});
dopromise.then(function(done){
     console.log()
}).catch(function(){
 console.log()
)}

But I don't understand properly how promises work in below code scenario. Code looks much simpler without details. Have I missed something?

eg 

navigator.mediaDevices.getUserMedia({
                 video: true;
 })
  .then(function(stream){
       alice.addStream(stream);
       return alice.createOffer();
  })

Upvotes: 0

Views: 98

Answers (3)

Storm
Storm

Reputation: 4445

In very general / (computer) layman terms promises are functions which get executed "asynchrounsly" and therefore when they return any information, there should be some place where you can capture it.

The information returned is captured by a "callback function" called then.

This callback is invoked automatically (by the javascript engine) and "takes a function" as an argument, which is then called again automatically :)

So if I had a piece of code like this

somePromise().then(function() { console.log('2 - Promise returned 
something, executing in a separate thread') });

console.log('1 - This is executed in the main thread');

The output would technically be:

1 - This is executed in the main thread
2 - Promise returned something executing in a separate thread

Because the promise can take a long time to return depending on how much work it is doing and you don't want the main thread to wait that long.

Upvotes: -2

Code-Apprentice
Code-Apprentice

Reputation: 83527

The syntax is the same. Whatever is before .then() must produce a Promise. In your case, you created the Promise explicitly. In the case of 

navigator.mediaDevices.getUserMedia({
    video: true;
})

getUserMedia() takes an object as a parameter and returns a Promise.

The rest is not legal JavaScript syntax as far as I know. You can either do

.then(function(stream){
   alice.addStream(stream);
   return alice.createOffer();
})

or with ES6

.then((stream) => {
   alice.addStream(stream);
   return alice.createOffer();
})

Upvotes: 1

Daniel A. White
Daniel A. White

Reputation: 190945

getUserMedia returns an already created Promise. In your first example you are manually creating a Promise in order to make something asynchronous.

Upvotes: 4

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